Cuprous complexes and dioxygen, VII . Competition between one‐ and two‐electron reduction of O 2 in the autoxidation of Cu(1‐methyl‐2‐hydroxymethyl‐imidazole) 2 +

The complexation of 1‐methyl‐2‐hydroxymethyl‐imidazole (L) with Cu(I) and Cu(II) has been studied in aqueous acetonitrile (AN). Cu(I) forms three complexes, Cu(AN)L + , CuL 2 + , and Cu(AN)H −1 L, with stability constants log K (Cu(AN) + + L ⇌ Cu(AN)L + ) = 4.60 ± 0.02, logβ 2 = 11.31 ± 0.04, and log K (Cu(AN)H −1 L+H + ⇌ Cu(AN)L + ) = 10.43 ± 0.08 in 0.15 M AN. The main species for Cu(II) are CuL 2+ , CuH −1 L + , CuH −1 L 2 + , and CuH −2 L 2 . The autoxidation of CuL 2 + was followed with an oxygen sensor and spectrophotometrically. Competition between the formation of superoxide in a one‐electron reduction of O 2 and a path leading to H 2 O 2 via binuclear (CuL 2 ) 2 O   2 2+was inferred from the rate law\documentclass{article}\pagestyle{empty}\begin{document}$${{ - {\rm d}\left[{{\rm O}_2 } \right]} \mathord{\left/ {\vphantom {{ - {\rm d}\left[{{\rm O}_2 } \right]} {{\rm dt}}}} \right. \kern-\nulldelimiterspace} {{\rm dt}}} = \left[{{\rm CuL}_2^ + } \right]^2 \left[{{\rm O}_2 } \right]\left({\frac{{k_{\rm a} }}{{1 + k_{\rm b} \left[{{\rm CuL}^ + } \right]}} + \frac{{k_{\rm c} \left[{\rm L} \right] + k_{\rm d} + {{\left({{{k_{\rm f} } \mathord{\left/ {\vphantom {{k_{\rm f} } {\left[{\rm L} \right]}}} \right. \kern-\nulldelimiterspace} {\left[{\rm L} \right]}} + k_{\rm g} + k_{\rm h} \left[{\rm L} \right]} \right)} \mathord{\left/ {\vphantom {{\left({{{k_{\rm f} } \mathord{\left/ {\vphantom {{k_{\rm f} } {\left[{\rm L} \right]}}} \right. \kern-\nulldelimiterspace} {\left[{\rm L} \right]}} + k_{\rm g} + k_{\rm h} \left[{\rm L} \right]} \right)} {\left[{{\rm H}^ + } \right]}}} \right. \kern-\nulldelimiterspace} {\left[{{\rm H}^ + } \right]}}}}{{\left[{{\rm CuL}_2 ^ + } \right] + k_{\rm e} \left[{{\rm Cu}\left({{\rm II}} \right)} \right]_{{\rm tot}} }}} \right)$$\end{document} with k a = (2.31 ± 0.12) · 10 4 M −2 S −1 , k b = (1.0 ± 0.2) · 10 3 M −1 , k c = (2.85 ± 0.07) · 10 2 M −2 S −1 , k d = 3.89 ± 0.14 M −1 S −1 , k e = 0.112 ± 0.004, k f = (2.06 ± 0.24) · 10 −10 M S −1 , k g = (1.35 ± 0.07) · 10 −7 S −1 , and k h = (6.8 ± 1.4) · 10 −7 M −1 S −1 .